Spreaders of this type are known from ports and terminals where they are used for transferring shipping containers from ships to road- or railroad vehicles, and vice versa. Typically, the spreader forms a part of an arrangement that further comprises cable sheaves by means of which the spreader is supported from a crane that operates and controls the moves of the spreader, and the spreader may be detachably coupled to the arrangement. Among the typical spreader movements are lowering of the spreader for coupling the spreader to one or several shipping containers, lifting and lowering of the spreader and container together, and horizontal shifting of spreader and container from one storage location to another. The invention is however not limited to spreaders supported in cables, but can also be applied to a spreader that is mounted in a lifting arm.
In order for connecting a container detachably to the spreader, the container comprises in each of its upper corners a seat shaped as an oblong hole that is open upwards. A head formed in a lower end of the locking pin can be inserted through the hole in an unlocking position of the locking pin. When inserted in the seat, rotation of the locking pin through about 90° into a locking position causes the head to engage from below the edge area about the oblong hole, this way connecting the spreader to the container. Unlocking is accomplished by turning the locking pin back to its original position.
Rotation of the locking pins between locking and unlocking positions is usually accomplished via push rods driven by means of hydraulic cylinders. The push rods are hingedly connected to the locking pins and arranged for simultaneous rotation of locking pins operated in pairs. The rotational position of the locking pins is typically controlled by means of sensors or limit switches that control the operation of the cylinder.
A sensor controlled operation of the rotational movements of the locking pins may however involve a risk of inaccurate positioning of the locking pins in the locking and unlocking positions. Deposits of dirt, wear, and play in mountings or interference with the sensors or control signals are factors that in course of time may cause displacement of the end positions in locking and unlocking modes of the locking pins, which in turn may lead to less secure connections between the spreader and the shipping container.